The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Internal combustion engines combust an air/fuel (A/F) mixture within cylinders to drive pistons and generate drive torque. A ratio of air to fuel in the A/F mixture may be referred to as an A/F ratio. The A/F ratio may be regulated by controlling at least one of a throttle and a fuel control system. The A/F ratio, however, may also be regulated by controlling other engine components (e.g., an exhaust gas recirculation, or EGR, system). For example, the A/F ratio may be regulated to control torque output of the engine and/or to control emissions produced by the engine.
The fuel control system may track a trajectory of a signal corresponding to a desired A/F ratio. The trajectory, however, may affect disturbance rejection performance and/or emissions reduction. For example, the trajectory may be a periodic sinusoidal signal. Therefore, the fuel control system may include an inner feedback loop and an outer feedback loop to improve tracking of the trajectory while maintaining disturbance rejection performance.
More specifically, the inner feedback loop may use data from an exhaust gas oxygen (EGO) sensor located upstream from a catalytic converter in an exhaust system of the engine system (i.e., a pre-catalyst EGO sensor). The inner feedback loop may use the data from the pre-catalyst EGO sensor to control a desired amount of fuel supplied to the engine (i.e., a fuel command).
For example, the inner feedback loop may decrease the fuel command when the pre-catalyst EGO sensor senses a rich A/F ratio in exhaust gas produced by the engine (i.e., non-burnt fuel vapor). Alternatively, for example, the inner feedback loop may increase the fuel command when the pre-catalyst EGO sensor senses a lean A/F ratio in the exhaust gas (i.e., excess oxygen). In other words, the inner feedback loop may maintain the A/F ratio at or near an ideal A/F ratio (e.g., stoichiometry, or 14.7:1), thus increasing the fuel economy of the engine and/or decreasing emissions produced by the engine.
Specifically, the inner feedback loop may perform proportional-integral (PI) control to correct the fuel command. Moreover, the fuel command may be further corrected based on a short term fuel trim or a long term fuel trim. For example, the short term fuel trim may correct the fuel command by changing gains of the PI control. Additionally, for example, the long term fuel trim may correct the fuel command when the short term fuel trim is unable to fully correct the fuel command within a desired time period.
The outer feedback loop, on the other hand, may use information from an EGO sensor arranged after the catalytic converter (i.e., a post-catalyst EGO sensor). The outer feedback loop may use data from the post-catalyst EGO sensor to correct (i.e., calibrate) an unexpected reading from the pre-catalyst EGO sensor, the post-catalyst EGO sensor, and/or the catalytic converter. For example, the outer feedback loop may use the data from the post-catalyst EGO sensor to maintain the post-catalyst EGO sensor at a desired voltage level. In other words, the outer feedback loop may maintain a desired amount of oxygen stored in the catalytic converter, thus improving the performance of the exhaust system. Additionally, the outer feedback loop may control the inner feedback loop by changing thresholds used by the inner feedback loop in determining whether the A/F ratio is rich or lean.
Exhaust gas composition (e.g., A/F ratio) may affect the behavior of the EGO sensors, thereby affecting accuracy of the EGO sensor values. As a result, fuel control systems have been designed to operate based on values that are different than expected. For example, fuel control systems have been designed to operate “asymmetrically.” In other words, for example, the error response to a lean A/F ratio may be different than the error response to a rich A/F ratio.
The asymmetry is typically designed as a function of engine operating parameters. Specifically, the asymmetry is a function of the exhaust gas composition, and the exhaust gas composition is a function of the engine operating parameters. The asymmetry is achieved indirectly by adjusting the gains and the thresholds of the inner feedback loop, requiring numerous tests at various engine operating conditions. Moreover, this extensive calibration is required for each powertrain and vehicle class and does not easily accommodate other technologies, including, but not limited to, variable valve timing and lift.